Purpose

This population-based, nested case-control study investigated the risks of second primary cancers of the ovary, endometrium, and contralateral breast among women receiving tamoxifen for breast cancer in conventional medical practice.

Methods

A cohort of women diagnosed with breast cancer during 1978 through 1990 was identified from a population-based cancer registry.

Case subjects included all women in the cohort who subsequently developed second primary ovarian (n=39), endometrial (n=42), or contralateral breast (n=234) cancer prior to 1992.

Results

The percentage of women who had received tamoxifen was 18% and 20%, respectively, among ovarian cancer case subjects and control subjects ; 26% and 31%, respectively, among endometrial cancer case subjects and control subjects ; and 10% and 18%, respectively, among contralateral breast cancer case subjects and control subjects.

The mean duration of tamoxifen use was less than 2 years for all groups.

Conclusions and Implications

These data suggest that short durations of tamoxifen therapy are not associated with an increased risk of endometrial or ovarian cancer but are associated with a reduction in contralateral breast cancer risk.

It would not be appropriate, however, to generalize these results to women who receive tamoxifen for longer periods.